Plural motor driving system adapted for regenerative braking

ABSTRACT

A driving system adapted for regenerative braking having two DC motors each having two series wound field windings. The motors and one field winding from each motor are connected in a bridge circuit to a DC source to provide for changing from motor to generator conditions. the other two field windings are connected in series with each other and with the DC source in a diagonal of the bridge. The motors are controlled in response to the average value of motor current.

United States Patent [72] Inventors Lars Goran Erikmon;

Karl-Gunner Goliath; Bo Ingemar Larsson, all of Vasteras, Sweden [21Appl. No 30,095

[22] Filed Apr. 20, I970 [45] Patented Aug. 24, 1971 [73] AssigneeAllrnanna Svenska Elektriska Aktiebolaget Vasteras, Sweden [32] PriorityApr. 22,1969

[3 3 1 Sweden [54} PLURAL MOTOR DRIVING SYSTEM ADAPT ED ron REGENERATIVEBRAKING 1 Claim, 2 Drawing Figs.

52 us. c1 318/87, 3l8/l 12, 318/376, 318/527 511 1m. 01 1102 7/70 50]Field olSearch ..3l8/49,81,

[56] References Cited UNITED STATES PATENTS 1,726,276 8/l929 Osborne318/87 3,069,606 12/1962 Pastoret et al. 318/81 X Primary Examiner-GrisL. Rader Assistant Examiner-H. Huberfeld Attorney-Jennings Bailey, Jr.

ABSTRACT: A driving system adapted for regenerative braking having twoDCmotors each having two series wound field windings. The motors and onefield winding from each motor are connected in a bridge circuit to a DCsource to provide for changing from motor to generator conditions. Theother two field windings are connected in series with each other andwith the DC source in a diagonal of the bridge. The motors arecontrolled in response to the average value of motor current.

PLURAL MOTOR DRIVING SYSTEM ADAPTED FOR REGENERATIVE BRAKING BACKGROUNDOF THE INVENTION the current source during braking by means of arectifier and I arranged in such a way that it can be short-circuitedpulsewise by means of a periodically operating electric switchingdevice, the length and/or frequency of the short-circuiting intervalbeing regulated in dependence on the average value of the total machinecurrent.

2. The Prior Art It is known, for instance through the French patent No.

1,337,643, to regulate the power from a' parallel-connected group of DCseries machines to a DC network by series-connecting the group with aresistor and connecting the series group thus obtained to the networkover a rectifier and to provide the series group with aparallel-connected short-circuiting circuit. which includes an electricswitching device in tended to close and open periodically theshortcircuiting circuit, whereby the length and/or frequency of theshort-circuiting interval is controlled in dependence on the averagevalue of the machine current. Each time a short-circuiting interval isfinished, the current continues to flow with unaltered direction throughthe series machines, but to the network instead of through theshort-circuiting circuit, and the e.m.f. required for this is generatedin said series group through selfinduction. In the regenerative intervalthere is no need of the generator voltage generated in the normal way inthe machines. On the contrary, it is an advantage if this is asinsignificant as possible. It is most important that the voltagegenerated in this series group does not become noticeably higher thanthe network voltage, which would cause impermissibly great currentstrength in the machines. The abovementioned resistor which is includedin the series group has the task of limiting the generator voltagedeveloped in the series group so that regenerative braking can beeffected even at high speed, but this means that a considerable portionof the braking energy is wasted. In a driving system according to theinvention, this disadvantage is avoided since only one part of the fieldwinding is used for generator operation.

When parallel-connected series motors or parallel-connected seriesgroups of such motors are to be driven as generators with the object ofeffecting regenerative braking, complicated connecting operations arerequired. First of all, the terminals must be interchanged on the fieldwinding or the armature winding. Furthermore, steps must be taken toensure even load distribution upon generator operation, for examplecross-connection so that the field windings of one parallel group havecurrent flowing through them from the other parallel group.

SUMMARY OF THE INVENTION The DC machines according to the invention areprovided with two series windings each. During braking only one serieswinding from each machine is then used and during driving the other isalso used. Thus, in addition to the requirements mentioned above, thereis also a need to be able to vary the cooperation between the variousparts of the field windings by means of connecting operations,corresponding to the need for regrouping upon transition from driving tobraking. However, 'since the different components of the equipment arearranged in a bridge connection in a special manner, the requiredconnecting operations are performable with a minimum of connectingdevices.

A driving system according to the invention is characterized in that themachines are provided individually with a first and a second serieswinding and that their rotors are evenly distributed on two oppositesides of a four-sided bridge, and said first series windings are evenlydistributed on the other sides of the bridge, and that the currentsource by means of switching means is connectable in a first diagonal ofthe bridge during operation and in a second diagonal during braking,whereas said second series windings are always included in said firstdiagonal.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in thefollowing with reference to the accompanying schematical drawings inwhich the embodiment shown in FIG. 1 illustrates the main principle ofthe invention and FIG. 2 shows a driving system according to theinvention provided-with control means intended to con trol the drivingtorque and the braking in an electric vehicle.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, 1 an 2 designate therotors in two DC series machines which are provided with two fieldwindings each. The two field windings belonging to the rotor 1 aredesignated 1a an lb and those included in the same machine as the rotor2 are designated 2a and 2b. The terminals of a DC source for the drivingsystem are designated 3 and 4. The arrangement includes a rectifier 5intended to conduct current when the DC machines are driven as motors,and two rectifiers 6 and 7 intend to conduct current during regenerativebraking, Furthermore, the driving means includes a circuit breaker 8intended to be closed .during motor operation and open during generatoroperation of the DC machines, and a circuit breaker 12 intended to beclosed only during generator operation. Nine is an electric switchingdeviceintended to break and close alternately at a high rate. Itsfrequency and the length of the various intervals are regulated by meansof the control device 10 which is supplied with the difference between areference value 15 and the sum of the output values from the currentemitters 13a and 13b connected in the rotor circuits, through acomparison device 16. Switching device 9 is thus controlled in responseto variations in a variable of the circuit. During motor operation theseries group 2,2a is parallel-connected with the series group 1,1a.During generator operation the series group 1, 2a is parallel-connectedwith the series group 2,212.

The direction of rotation and also the induced electromotoric power ofthe DC machines remain unaltered upon switching over from motoroperation to braking, but the rotor current on the other hand changesdirection. It is therefore necessary that the field winding, uponswitching over between motor operation and braking, be arranged in sucha way in relation to the rotor winding that altered current direction inthe latter does not cause an alteration in the direction of theexcitation flux. As is clear from the drawings, this is achieved in thearrangement shown. The arrows 11 indicate the direction of theabove-mentioned electromotoric power and in the rectifiers 5,6,7 thecurrent direction is indicated in the drawing by the arrowlike part ofthe graphic symbols.

The various components of the device are connected together in afour-branch bridge and each branch is shown in the drawing as a side ofa rectangle. The rotors l and 2 are included one in each of two oppositesides and the field windings 2a and 2b one in each of the other sides ofthe rectangle in such a way that the two motors are parallel-connectedacross the diagonal BD, only windings belonging to one and the samemotor being connected in each parallel circuit, whereas each of the twoparallel -connected circuits which are connected between the diagonalend points A and C include a rotor and a series winding belonging toseparate machines. When the latter parallel-connection is used aso-called cross-connection" is obtained, which is a well-knownprecaution to ensure an even load distribution upon parallel-connectionof two series generators.

During motor operation the total current delivered to the driving meansfrom the terminal 3 flows over the breaker 8 through the two fieldwindings 1b and 2b and divides in the diagonal end point B into twoparallel circuits, one of which consists of the rotor 2 and its fieldwinding 2a. whereas the other consists of the rotor l and field windingla. From-the diagonal end point D the total current is conducted throughthe current converter 5 and electric switching device 9 to the negativeterminal'4 of the current source, in an interval when the electricswitching device continues to flow for a short while to theautoinduction in the windings of the machine and is closed through theconnection 14 and current converter 7. The average value of the motorcurrent is determined by the value set by the reference value device 15.

Upon switching over to braking thebreaker 8 is opened, as mentioned, andthe breaker 12 is closed. Because of the remanence in the field windings1a and 2a, a generator voltage is generated, the direction of thisvoltage still coinciding with the direction of the arrows 11. If theelectric switching device 9 is closed, therefore, a short-circuitingcurrent arises in each of the two-parallel-connected machine circuits;One of these short-circuiting currents flows from the rotor 1, throughthe breaker 12, communication 14, electricswitching device 9, currentconverter 6 and field winding 2a back to the rotor l. The othershort-circuiting current flows from the rotor 2 through the fieldwinding 1a, breaker 12, connection 14, electric switching device 9,current converter 6 and back to the rotor 2. When the electric switchingdevice 9 is opened, the current flows with the same direction throughthe rotors l and 2 and the field windings 1a and 2a and is now fed in tothe terminals 3 and 4 of the direct current source, while an e.m.f.exceeding the voltage of the direct current source is generatedsubstantially by means of self-induction in the field windings.

1a and 2a, these having so few turns that their excitation flux--even atthe highest possible speed-is only able to generate generatorvoltagewhich is considerably below the voltage of the DC source. Theaverage value of the machine current is determined by the magnitude ofthe reference value furnished at 15 and in such a way that the machinesare not over loaded.

In the embodiment of the invention shown in FIG. 2, the

electric switching device 9 consists of a thyristor which is pro videdwith equipment, known per se, for rapidly alternating ig- I nition andextinction. Furthermore, FIG. 2 differs from FIG. 1 in that a reactor 20is included in the driving system. The designations used in FIG. 1 arealso used in FIG. 2 and have the same meaning. The breakers 8 and 1 2are shown in position for regenerative braking. The thyristor 9 must becurrentless for about 20 1. sec. if it is to be able to block afterhaving been conducting.

This is made possible with the help of a circuit containing twothyristors 21 and 22, a capacitor 23 and a reactor 24. The thyristors 9and 21 are given ignition impulses simultaneously, whereby the capacitor23, which has positive voltage in relation to the bar 4, discharges inthe form of a current through V thyristor 21, reactor 24 and thyristor9. The sinusoidal current which thus arises on the oscillating circuitformed by the reac-' tor 24 and the capacitor 23 dies out only after thefirst half period, since the thyristors'21-and 9 do not permit thiscurrent to change direction. The capacitor 23 will, during the course ofthis half-period, be charged to opposite polarity. The thyristor 22receives its ignition impulse somewhat later than the thyristors 21 and9, and, when this occurs, the capacitor 23 will once again be rechargedso that it acquires its original; polarity. The capacitor 23 isthusparallel-connected over the for the thyristor 22 assing the delay means19, the delay being dependent ont e control magnitude delivered from thecomparison device 16. The desired average value of the machine currentis set by means of the reference value device 18. i 4 i The oscillator25 and the delay device 19 may be of any known design, for exampleinaccordance with FIGS. -3 and 4,

respectively of .Persson Pat. application S.N. 787,180, filed Dec. 26,1968.

The current transducer 13 may quite simply consist of a series-connectedresistor in the same way as the item 6 in the drawing in Swissspecification No 477,306, and the comparison device 16 may then in itsmost simple form consist of a connecting device for series connectingthe output voltages from the devices 13 and 16 in opposition to eachother, the device being furnished with output terminals for theresultant voltage.

We claim: 1. Driving system of regenerative braking comprising a bridgecircuit having first, second, third and fourth sides (AB,

BC, CD, DA) andfirst and second diagonals (AC BD), the

first and third sides being opposite each other, two DC series machineshaving their rotors (1,2) connected in the second and .fourth sides,respectively, each machine having first and second field windings (1a,1b, 2a, 2b), the first field windings (1a,2a) beingconnected in thefirst-and third sides, respectively, a current source (3,4)switchingmeans (8,12) for con- 7 nectingsaid current source in thesecond diagonal during 4 motor operation and in the first diagonalduring braking, the

second field windings being connected in the second diagonal, rectifiermeans (6,7) in each of said diagonals, a periodically operatingswitching device (9) for short-circuiting said rectifi- 7 er means, andmeans (10) responsive to variations in a variable of said circuittocontrol the operation of said switching device. a

1. Driving system of regenerative braking comprising a bridge circuithaving first, second, third and fourth sides (AB, BC, CD, DA) and firstand second diagonals (AC BD), the first and third sides being oppositeeach other, two DC series machines having their rotors (1,2) connectedin the second and fourth sides, respectively, each machine having firstand second field windings (1a, 1b, 2a, 2b), the first field windings(1a,2a) being connected in the first and third sides, respectively, acurrent source (3,4) switching means (8,12) for connecting said currentsource in the second diagonal during motor operation and in the firstdiagonal during braking, the second field windings being connected inthe second diagonal, rectifier means (6,7) in each of said diagonals, aperiodically operating switching device (9) for short-circuiting saidrectifier means, and means (10) responsive to variations in a variableof said circuit to control the operation of said switching device.